GB-2,170,409 describes a breathing apparatus including a gas passage having an inlet to receive gas, an outlet to pass gas to the patient and a valve in the passage to control the flow of gas to provide the desired pulsed flow of gas. A variable throttle is included downstream of the valve which, when gas is flowing in the passage, diverts part of it into a reservoir. The reservoir includes a leak valve which allows gas to leak from the reservoir to atmosphere and a check valve that prevents gas in the reservoir from returning to the gas supply system of the apparatus. The valve includes a piston for opening and closing the valve; the piston is biassed (a) by pressure in the reservoir to move in one direction and (b) by a spring to move in the opposite direction. When the valve is open, gas flows through the passage to the patient, who inhales; at the same time, gas is diverted into the reservoir from the passage at a faster rate than it leaks out of the reservoir through the leak valve and so the pressure in the reservoir increases until it reaches a value at which it can move the piston to close the main valve causing the flow of gas to the patient to the patient to cease, allowing him to exhale (expiratory period). Since there is no gas flowing through the passage, no gas is diverted into the reservoir and the leakage of gas through the leak valve to atmosphere causes the pressure in the reservoir to fall; when the pressure reaches such a low level that the force it exerts on the piston is less than that exerted by the spring, the spring moves the piston to open the valve and resume the flow of gas in the passage thereby causing the patient to inhale once more (inspiratory period). The cycle of the inspiratory expiratory periods are repeated for as long as required. In order to adjust the duration of the inspiratory and expiratory periods, the leak valve needs to be made as a needle valve and a further needle valve needs to be provided in the duct diverting gas into the reservoir.